My understanding is that when multiple authors create a manuscript they become joint copyright owners.
From my research, it appears that as a joint author you cannot grant exclusive licenses:
As a joint author, you cannot grant an exclusive license. Granting an exclusive license would directly harm the other joint authors, and thus you need their approval. See the section on Copyright Licenses and Transfers for more on the difference between exclusive and non-exclusive licenses.) However, you may grant non-exclusive licenses without approval from the other authors, which can lead to situations in which each author licenses the work to a different publisher.
However, some subscription journals only seek a contractual exclusive license or copyright transfer from the corresponding author. Since the other authors never sign an agreement, do they retain all the rights they possessed prior to any agreement between the publisher and corresponding author? If so, could a joint author who made no agreement release the typeset article under any non-exclusive license of their choosing, such as CC-BY?
For example take this License to Publish for IJE. Only the corresponding author completes the agreement. The only mention of coauthors comes on the signature line:
Signature (on behalf of all co-authors (if any))
The idea that the corresponding author can sign on behalf of all coauthors seems suspect to me.
Do any legal experts have an assessment of this situation? Also other instances where journals only directly engage the corresponding author for a copyright agreement would be appreciated.